The purpose of this study is to compare the concentrations of nitric oxide and its aqueous phase products, nitrite, nitrate, S-nitrosothiol, and ammonia in young children with CF vs. older children with CF vs. normal children and to analyze how they change after anti-pseudomonal therapy. Possible benefits include better assessment of the severity of airway inflammation, better ability to determine infection with a pathogenic bacteria, and potentially the development of new therapies for cystic fibrosis. Recruitment ongoing.